Polymer sheet for projection screen

ABSTRACT

A polymer sheet for use in a projection screen comprises polyorgano silsesquioxane microspheres that enhance light diffusing characteristics. Also disclosed is a projection screen comprising at least two glass plates and the polymer sheet disposed therebetween. Also disclosed is a method of manufacturing a polymer sheet by incorporating the microspheres with the polymer.

TECHNICAL FIELD

This invention relates to a polymer sheet, film or foil (hereinafterreferred to as a sheet) for use in a projection screen, such as alaminated rear projection screen. More particularly, the inventionrelates to a polymer sheet, such as a polyvinyl butyral sheet,comprising polyorgano silsesquioxane microspheres to improve lightdiffusing characteristics. The invention also relates to a projectionscreen comprising at least two glass plates and the polymer sheetdisposed therebetween.

BACKGROUND OF THE INVENTION

Projection screens, particularly rear, transmission, or back projectionscreens in which modulated light from a projector is illuminated on theside facing away from the viewer and that light is viewed on the sidefacing the viewer, are used mainly for presentations, for showingslides, films, and video, for television sets and the like.

U.S. Pat. No. 6,839,167, Eckelt, et al., describes a projection screenthat allows viewing of the projected image on the side of the screenfacing away from the projector as well as on the side facing theprojector. The screen is said to have only a low flicker effect and nolight spots or places with excessive gloss. The screen has a first glassplate facing the projector that has a translucent ceramic coating, asecond glass plate facing away from the projector that has a surfaceroughness to prevent reflections, and a white translucent polyvinylbutyral (PVB) sheet therebetween that is said to provide dispersion andpartial reflection of the light to achieve best possible reproduction ofthe image. However, designing the screen to both reflect and transmitlight so as to allow viewing from both sides degrades the brightness andcontrast of the image to the viewer. It is also not practical to viewprojected information and data such as text when it is displayedbackwards, as would be the case depending on which side of the screenyou are viewing.

In the case of a rear projection screen, it is desirable to have highlight transmission by scattering and diffusing forward as much light aspossible while limiting the amount of light reflection backwards. It isalso desirable that the light be spread over a wide viewing area with novisible concentration or hotspot when viewing the screen. It is alsodesirable to have the projection screen limit ambient light reflectingoff the screen so as not to degrade the image contrast. The projectionscreen must also be able to resolve the individual picture elements orpixels projected. Thicker sheets of polymer (e.g., greater than about0.060 inches [about 1.524 mm] thick) such as those available for lightdiffusion with diffusion particles dispersed throughout the sheet appearmilky and will not exhibit sufficient resolution or contrast. It isdesirable that the screen be easy and inexpensive to manufacture as wellas scalable to very large sizes. The screen should also be durable, easyto clean, and able to withstand outdoor exposure.

Advances in projector resolutions are starting to render screen designs,such as plastic lenticular sheets disclosed in U.S. Pat. No. 4,919,515,Hasegawa, et al., more difficult and expensive to manufacture. The lensstructure and pitch must be made smaller to accommodate higherresolutions; and they are also prone to moiré, speckle and other screenartifacts. Rear projection screens of the glass beaded type, such asdisclosed in U.S. Pat. No. 2,378,252, Staehle, et al., while having goodbrightness and contrast suffer from an excess amount of mottle or blotchdue to variations in the bead diameters. This causes the bead to haveincomplete depth in the light absorbing layer and uneven lighttransmission across the screen. Surface relief holograms, such asdisclosed in U.S. Pat. No. 5,609,939, Petersen, et al., while being highin light transmission and able to spread projected light over a widearea, are difficult and expensive to manufacture in larger sizes and areprone to pin hole defects causing excessive bright spots where the lightis not diffused.

Many current PVB sheets designed to diffuse light do so by incorporatingfine particles or pigments of a different refractive index, such ascalcium carbonate, barium sulfate, silica, or quartz, into the PVB resinmatrix. Particle size, morphology of the particle, concentration ofparticle to carrier resin, and the ratio of the refractive indices ofthe particle and the carrier resin all determine the quantity ofscattered light. Typically, the particle size is between about 0.1 andabout 10 microns. Too small of a particle size will become transparentto light and will not diffuse. Too large of a particle size anddiffusion efficiency diminishes. Spherical particles are preferred asthey exhibit a constant size to the projected light no matter what theirorientation is within the carrier resin. This results in a more uniformand smooth appearance to the screen. If the contrast of the refractiveindices is excessive, more light is reflected back and less is scatteredforward. If the contrast is too low, the light is transmitted withoutbeing sufficiently scattered or diffused. Typically, the difference inrefractive indices between the particles and the carrier resin is fromabout 0.06 to about 0.14. Particles of a lower refractive index than theresin are preferred. Particles having a higher index than the carrierresin will exhibit more light backscatter. The pigments should also betransparent or translucent so as not to be too opaque to light.Optically dense pigments such as titanium dioxide have a significantamount of light reflection and do not sufficiently scatter and diffusethe light forward. Some pigments are also difficult to disperse into thepolymer resin matrix, or have a high specific gravity and settle out ofdispersion. In other pigments, the morphology of the particle cansignificantly raise the viscosity of the resin matrix and makeprocessing difficult. Selection of both pigment and carrier resin thatmaintains polarity of projected light is also desirable. There areapplications with projection displays that utilize polarization of lightto separate modulated light signals to two channels or views, one forthe left and one for the right eye to show 3D images to the viewer. Apolarization extinction ratio of at least about 25:1 is desirable tominimize ghosting or leaking from one channel to the other channel.Extinction ratios of 100:1 and above are preferred. Polymers such aspolycarbonate or polyethylene terephthalate (PET) will depolarize lightas will pigment such as calcite crystals.

Thus, there is a continuing need for improved polymer sheets thatprovide good light diffusion with an even appearance and minimal screenartifacts when used in projection screens.

SUMMARY OF THE INVENTION

The invention relates to a polymer sheet comprising from about 85% toabout 99.5% by weight of a thermoplastic polymer compound and from about0.5% to about 15% by weight of dispersed polyorgano silsesquioxanemicrospheres.

The invention also relates to a projection screen comprising:

a) a first glass plate;

b) a second glass plate; and

c) a polymer sheet layer disposed between the first glass plate and thesecond glass plate, said polymer sheet comprising from about 85% toabout 99.5% by weight of a thermoplastic polymer compound and from about0.5% to about 15% by weight of dispersed polyorgano silsesquioxanemicrospheres.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing, a projection screen according to the invention isrepresented schematically in a partly sectioned side view.

DETAILED DESCRIPTION OF THE INVENTION

The polymer sheet of the present invention comprises from about 85% toabout 99.5%, typically from about 90% to about 99%, by weight of athermoplastic polymer compound having a suitable glass transitiontemperature. Polymer sheets useful herein are described in U.S. Pat. No.6,825,255, Yuan et al., incorporated herein by reference. In oneembodiment, the polymer sheet comprises polyvinyl butyral, polyurethane,polyvinyl chloride, or poly (ethylene vinyl acetate), or combinationsthereof. The sheet typically comprises polyvinyl butyral, polyvinylchloride, or polyurethane, or mixtures thereof, especially polyvinylbutyral (PVB).

PVB is produced by known aqueous or solvent acetalization processes thatinvolve reacting PVOH with butyraldehyde in the presence of acidcatalyst, followed by neutralization of the catalyst, separation,stabilization and drying of the resin. The polymer typically comprisesabout 13% to about 30% by weight of hydroxyl groups calculated as PVOH,more typically about 15% to about 22% hydroxyl groups calculated asPVOH. The polymer may further comprise up to about 10% by weight ofresidual ester groups, typically up to about 3% residual ester groups,calculated as polyvinyl acetate, with the balance being an acetal, e.g.,butyraldehyde acetal, but optionally including other acetal groups,e.g., a 2-ethyl hexanal-group. Typically, the PVB has an averagemolecular weight greater than about 70,000 g/mole. PVB is commerciallyavailable from Solutia Inc., St. Louis, Mo. as Butvar resin.

The polymer sheets herein have improved light diffusing characteristicsdue to the addition of an effective amount of polyorgano silsesquioxanemicrospheres. The organo groups in the microspheres can be methyl,C₂-C₁₈ alkyl, hydride, phenyl, vinyl, or cyclohexyl, or mixturesthereof. Suitable microspheres are described in U.S. Pat. No. 6,773,787,Maas et al., and in U.S. Pat. No. 5,352,747, Ohtsuka et al., bothincorporated herein by reference. The microspheres can be prepared byconventional methods, such as disclosed in F. Brown et al., J. PolymerSci., Part C, No. 1, p. 83 (1983), in which one or more of thetrialkoxysilanes are hydrolyzed with an acid catalyst and condensed.

In one embodiment, the polyorgano silsesquioxane comprises methyl,C₂-C₁₈ alkyl, hydride, phenyl, vinyl, or cyclohexyl groups, or acombination thereof. Examples include polymethyl silsesquioxane,polyphenyl silsesquioxane, polyphenyl-methyl silsesquioxane, a phenylsilsesquioxane-dimethyl siloxane copolymer in liquid form,polyphenyl-vinyl silsesquioxane, polycyclohexyl silsesquioxane,polycyclopentyl silsesquioxane, and polyhydride silsesquioxane, andcombinations thereof.

In another embodiment, the polyorgano silsesquioxane is a polyalkylsiloxane powder material prepared by hydrolysis, polymerization orcrosslinking of alkylsilanes or alkylsiloxanes in such a way as to givea defined particulate structure with a surface consisting largely ofalkylfunctional silicone atoms. In yet another embodiment, the siliconladder resin is a poly (methyl silsesquioxane) obtained by hydrolyticcondensation in aqueous ammonia or amines of methyltri-alkoxysilanes, ortheir hydroxylates or condensates. The resin is spherical in shape andforms free-flowing powders, which are low in impurities such aschlorine, alkali metals, or alkaline earth metals.

The polyorgano silsesquioxane microspheres are used in the polymer sheetin a sufficient amount to provide the desired light diffusingproperties. In one embodiment, the sheet comprises from about 0.5% toabout 15%, typically from about 1% to about 10%, by weight of themicrospheres.

The polyorgano silsesquioxane microspheres typically have an averageparticle size of from about 0.1 to about 10 microns, more typically fromabout 0.5 to about 7.0 microns. A tight particle size distribution isdesired for improved visual appearance and performance. In oneembodiment, the microspheres are polymethyl silsesquioxane powderavailable from Toshiba Silicones under the trade name Tospearl 145, witha mean particle size of about 4.0 microns. In another embodiment, themicrospheres are available from Toshiba Silicones under the trade nameTospearl 120, with a mean particle size of about 2.0 microns. Therefractive index is about 1.42

The polymethyl silsesquioxane microspheres herein have a low specificgravity and are easily dispersed in the polymer matrix. Moreover, theyhave a spherical shape and are free flowing so they do not significantlyraise the viscosity of the polymer resin matrix during processing andforming into a sheet. The microspheres are efficient at diffusing lightforward, but do not significantly reflect light back towards the lightsource. Low concentrations are often sufficient to completely diffusethe light so that one viewing the glass laminate in front of the lightsource does not see a significant concentration of light on the screen.It is important that the viewer when looking on axis to the projectorlight does not detect the position of the light source behind thescreen. The microspheres are typically resistant to high temperatures,solvents, and plasticizers, and do not react with the chemistry of thePVB matrix to cause yellowing or other discolorations of the sheet. Themicrospheres are also not abrasive and do not cause excessive wear toprocessing equipment as some other pigments do. The resulting polymersheet can be used in a rear projection screen or other applications thatrequires a light-diffusing panel, such as in skylights or privacyscreens.

The polymer sheets of the present invention may further comprise anyadditive typically used in such sheets, in conventional amounts. In oneembodiment, such additives are present in an amount of from about 0.1%to about 30% by weight of the composition. The use of such additives maybe desirable for improved processing of the composition as well asimproving the products or articles formed therefrom. Examples of suchadditives include oxidative and thermal stabilizers, lubricants, moldrelease agents, flame-retarding agents, oxidation inhibitors, dyes,pigments and other coloring agents, ultraviolet light absorbers andstabilizers, nucleators, plasticizers, compatible plastics, anti-staticagents, fillers, and other conventional additives known in the art, andmixtures thereof. The additives may be incorporated at any suitablestage of the production process, and typically are introduced in themixing step and included in an extrudate.

By way of example, representative ultraviolet light stabilizers includevarious substituted resorcinols, salicylates, benzotriazole,benzophenones, and the like. Suitable lubricants and mold release agentsinclude stearic acid, stearyl alcohol, stearamides. Exemplaryflame-retardants include organic halogenated compounds, includingdecabromodiphenyl ether and the like, as well as inorganic compounds.Suitable coloring agents, dyes and pigments include cadmium sulfide,cadmium selenide, titanium dioxide, phthalocyanines, ultramarine blue,nigrosine, carbon black and the like. Representative oxidative andthermal stabilizers include the Group I metal halides, such as sodiumhalides, potassium halides, and lithium halides, as well as cuproushalides, and also chlorides, bromides, and iodides, hindered phenols,hydroquinones, and aromatic amines, as well as substituted members ofthose above mentioned groups, and combinations thereof. Exemplaryplasticizers include lactams such as caprolactam and lauryl lactam,sulfonamides such as o,p-toluenesulfonamide and N-ethyl, N-butylbenylnesulfonamide, and combinations of the above. Other plasticizerscommonly employed are esters of a polybasic acid or a polyhydricalcohol. Suitable plasticizers include, for example, triethylene glycoldi-(2-ethylbutyrate), triethylene glycol di-(2-ethylhexanoate),triethylene glycol diheptanoate, tetraethylene glycol diheptanoate,dihexyl adipate, dioctyl adipate, hexyl cyclohexyladipate, mixtures ofheptyl and nonyl adipates, diisononyl adipate, heptylnonyl adipate,dibutyl sebacate, polymeric plasticizers such as the oil-modifiedsebacic alkyds, and mixtures of phosphates and adipates such asdisclosed in U.S. Pat. No. 3,841,890, and adipates such as disclosed inU.S. Pat. No. 4,144,217. Also commonly employed plasticizers are mixedadipates made from C₄ to C₉ alkyl alcohols and cyclo C₄ to C₁₀ alcohols,as disclosed in U.S. Pat. No. 5,013,779, e.g., C₆ to C₈ adipate esters,such as dihexyl adipate.

In one embodiment, the polymer sheet herein comprises, by weight, fromabout 60% to about 80% PVB, from about 15% to about 30% of plasticizer(e.g., triethyleneglycol bis(2-ethylhexanoate), and from about 0.5% toabout 15% of the polyorgano silsesquioxane microspheres (e.g.,polymethyl silsesquioxane microspheres).

The polymer, microspheres, and other additives herein are thermallyprocessed and configured into sheet form. One exemplary method offorming a PVB sheet comprises extruding molten PVB resin (hereinafter“melt”) comprising the microspheres and other additives by forcing themelt through a sheet die (e.g., a die having an opening that issubstantially greater in one dimension than in a perpendiculardimension). Another method of forming a PVB sheet comprises castingmolten resin or semi-molten resin comprising the microspheres and otheradditives from a die onto a roller, solidifying the resin, andsubsequently removing the solidified resin as a sheet. In eitherembodiment, the surface texture at either or both sides of the sheet maybe controlled by adjusting the surfaces of the die opening or byproviding texture at the roller surface. Other techniques forcontrolling the sheet texture include varying parameters of the reactantmaterials (e.g., the water content of the resin and/or plasticizer, themelt temperature, or combinations thereof). Furthermore, the sheet canbe configured to include spaced projections that define a temporarysurface irregularity to facilitate de-airing of the sheet duringlamination processes, after which the elevated temperatures andpressures of the laminating process cause the projections to melt intothe sheet, thereby resulting in a smooth finish. The sheet can be madein different thickness, for example, about 0.015 inches (about 0.381 mm)or about 0.030 inches (about 0.762 mm). The sheet can also be layeredduring lamination to yield thicker forms.

The above polymer sheet can be used in a projection screen made of alaminated glass having at least two glass plates, with the polymer sheetdisposed therebetween. For example, the drawing schematically shows aprojection screen of the invention in a partially sectioned side view.As shown in the drawing, projection screen 1 comprises two glass plates3, 4 bonded to each other by means of a layer of polymer sheet 2.Polymer sheet 2 is a translucent sheet of polyvinyl butyral comprisingpolymethyl silsesquioxane microspheres. The glass plate 3 facing aprojection device 5 has an outer surface 6 extending at least over theprojection area. The glass plate 4 facing away from the projectiondevice 5 has on its outer surface 7 a surface roughness preventingreflections. The glass plate 3 comprises of a glass that is low in ironoxide and the glass plate 4 comprises of silicate glass. In oneembodiment, the glass plate 3 is about 0.125 inches (about 3.175 mm)thick, and the glass plate 4 is about 0.125 inches (about 3.175 mm)thick. One or more additional glass plates, with one or more additionalpolymer sheets between the glass plates, may be added as desired.Thicker glass may also be used for additional structural strength. Thishas little effect on the optical performance of the screen.

In one example, a PVB sheet 0.015 inches (about 0.381 mm) thickcomprises about 10% by weight of Tospearl 120 to PVB resin weight, andabout 25-30% of triethyleneglycol bis(2-ethylhexanoate) plasticizer. Thesheet performs well in a rear projection screen, with a screen gain of1.3 and no hot spot or blow through of the projected light. In anotherexample, a 0.030 inch (about 0.762 mm) thick PVB sheet comprising about7% by weight of Tospearl 120 to PVB resin weight, and about 25-30% oftriethyleneglycol bis(2-ethylhexanoate) plasticizer performs well in arear projection screen. The projection screen has a peak gain of 1.1 anda ½ gain viewing angle of over 50 degrees. The Polarization ExtinctionRatio (PER) is about 33:1. By utilizing a first glass plate of tintedglass of about 61% visible light transmission, the PER is raised to over50:1. Adding a light absorbing pigment or dye to the polymer sheetraises the PER even higher, over 100:1. Resolution is still excellentwith the thicker sheet.

To improve the contrast of the projected image by absorbing ambientlight, it may be desirable to also include a light-absorbing layer inthe construction. This can be done with gray glass, additionaltransparent gray PVB layers, or tinted transparent gray films such asPET included in the layers. The translucent nature of the pigmented PVBsheet allows the projection screen to appear a dark neutral gray color.Other pigments with a higher amount of light backscatter will appearlighter in color, decreasing the contrast of the projection screen. Itis also possible to tint the sheet gray by including a light absorbingpigment such as carbon black in the PVB along with the microspheres. Inone example with the screen constructed with 3/16 inch (about 4.8 mm)thick gray glass (approximately 50% visible light transmission) as onelayer, only about 4.5% of the ambient light is reflected off the screenback to the viewer. A matte white front projection screen will reflectnearly 100% back to the viewer.

For rear projection screens, the concentration of the microspheresshould be high enough to diffuse the light so that the viewer cannotdiscern the projected light source. Thus, a PVB sheet 0.015 inches(about 0.381 mm) thick typically comprises from about 9% to about 10% byweight of the microspheres, and a sheet 0.030 inches (about 0.762 mm)thick typically comprises about 5% by weight of the microspheres.Increasing the concentration of microspheres will decrease the totallight transmission but at the same time it will increase the dispersionangle of the diffused light. This is desirable for rear projectionscreen applications that require edge blending or otherwise uniformbrightness. The glass used in the construction can also be enhanced byapplying an anti-reflection coating or tinted coating or by etching theglass slightly to reduce surface reflections on the glass that degradethe projected image. For example, the peak-to-valley height of a glassplate with surface roughness may lie in the range of from about 2 toabout 5 microns. As mentioned above, the addition of a gray contrastenhancement layer may also be beneficial. For other applications whereincreased light transmission is desired, lower concentrations of themicrospheres can be used.

The combination of the two glass plates with the polymer sheet disposedtherebetween prevents a concentrated point of light on the glasssurfaces from being produced during incident-light and transmitted-lightprojection, which would prevent a clear projection image. Instead, thecone of light produced by the projector on the glass surface isdispersed by the microspheres so that no disturbing concentrated pointof light is produced and the projected image can be clearly seen.Moreover, the projected image is not falsified in its color and ispresented in a uniformly sharp and undistorted way.

Various embodiments of this invention have been described. However, thisdisclosure should not be construed as a limitation on the scope of theinvention. Accordingly, various modifications, adaptations, andalternatives may occur to one skilled in the art without departing formthe spirit and scope of the claimed invention.

1. A polymer sheet comprising from about 85% to about 99.5% by weight ofa thermoplastic polymer compound and from about 0.5% to about 15% byweight of dispersed polyorgano silsesquioxane microspheres.
 2. Thepolymer sheet of claim 1 wherein the polymer sheet comprises polyvinylbutyral.
 3. The polymer sheet of claim 1 comprising from about 60% toabout 80% by weight of polyvinyl butyral.
 4. The polymer sheet of claim1 wherein the polyorgano silsesquioxane comprises methyl, C₂-C₁₈ alkyl,hydride, phenyl, vinyl, or cyclohexyl groups, or a combination thereof.5. The polymer sheet of claim 1 wherein the polyorgano silsesquioxane isa polymethyl silsesquioxane.
 6. The polymer sheet of claim 5 comprisingfrom about 1% to about 10% by weight of polymethyl silsesquioxanemicrospheres.
 7. The polymer sheet of claim 6 wherein the microsphereshave a mean particle size of equal to or less than about 4.0 microns. 8.A projection screen comprising: a) a first glass plate; b) a secondglass plate; and c) a polymer sheet layer disposed between the firstglass plate and the second glass plate, said polymer sheet comprisingfrom about 85% to about 99.5% by weight of a thermoplastic polymercompound and from about 0.5% to about 15% by weight of dispersedpolyorgano silsesquioxane microspheres.
 9. The projection screen ofclaim 8 wherein the first glass plate has a low iron oxide content. 10.The projection screen of claim 8 wherein the second glass plate issilicate glass.
 11. The projection screen of claim 8 wherein the polymersheet comprises polyvinyl butyral.
 12. The projection screen of claim 8comprising polymethyl silsesquioxane microspheres.
 13. The projectionscreen of claim 12 wherein the polymer sheet comprises polyvinylbutyral.
 14. The projection screen of claim 13 wherein the microsphereshave a mean particle size of equal to or less than about 4.0 microns.15. The projection screen of claim 14 comprising from about 1% to about10% by weight of polymethyl silsesquioxane microspheres.
 16. Theprojection screen of claim 15 wherein the first glass plate has a lowiron oxide content and the second glass plate is silicate glass.
 17. Theprojection screen of claim 16 wherein the second glass plate facing awayfrom the projection device has a rough outside surface that has apeak-to-valley height of between about 2 and about 5 microns.
 18. Aprojection screen comprising: a) a first glass plate; b) a second glassplate; and c) a polymer sheet layer disposed between the first glassplate and the second glass plate, said polymer sheet comprising fromabout 85% to about 99.5% by weight of a polyvinyl butyral compound andfrom about 0.5% to about 15% by weight of dispersed polymethylsilsesquioxane microspheres.
 19. The projection screen of claim 18wherein the microspheres have a mean particle size of equal to or lessthan about 4.0 microns.
 20. The projection screen of claim 19 whereinthe first glass plate has a low iron oxide content and the second glassplate is silicate glass.